Of apparatus using a beam of electrons or ions which are a kind of charged-particles, a charged-particle beam apparatus such as an electron microscope for focusing and irradiating electrons on a specimen surface to form an image thereon, an electron beam exposure apparatus or an ion beam apparatus has been playing an important role in a wide technology field. For focusing a charged-particle beam in these apparatus, an electron lens comprised of electrodes or magnetic poles is used. In using the electron lens as above, there arises a problem of electro-optical aberration. Generally, in the electron lens constructed rotationally symmetrically, the focusing function increases at off-axis locations closer to the magnetic poles, enabling the electron lens to function as a concave lens. Further, the interaction of an electromagnetic field with the charged-particle beam causes a higher-order perturbation component leading to an aberration by which a charged-particle beam emitted from a certain point is caused to undergo slight dispersion depending on an incident condition to the lens as is known in the art. For these reasons, even with an ideal point light source, its emission angle distribution and center trajectory axis affect the image forming point to spread it finitely, thus bringing about a so-called beam blur. This type of aberration causes a degraded resolution and a serious cause of accuracy degradation in specimen observation and in fine working, respectively, both using the focused charged-particle beam.
In estimation of aberration in the rotationally symmetric system, the perturbation theory shows that for a small incident angle α making to the rotationally symmetric axis, there occur a spherical aberration proportional to the third power of α and a chromatic aberration proportional to a deviation dV of accelerating energy −V and by neglecting off-axis and higher-order terms, a beam trajectory deviation δ on the axis can be expressed byδ=Csα3+Cc(dV/V)α  (1)where Cs represents a spherical aberration coefficient and Cc represents a chromatic aberration coefficient. Equation 1 indicates that in the presence of beam current distribution and energy dispersion, a beam blur dependant on α takes place on the axis. Generally, in the charged-particle beam apparatus, the wider the extraction of the charged-particle beam emitted from the light source, the more the trajectory distribution in the focusing lens spreads to increase the amount of aberration. In order to decrease the aberration amount, a method is adopted according to which the electron lens impersonating a source of aberration is excited intensively to make short the focal point and the working length as well so as to reduce the trajectory dispersion, that is, blur.
In an alternative method for correction of this type of aberration, an aberration correction method has been proposed in which many stages of rotationally asymmetric multiple poles are arranged with a view to controlling the dispersion and focus (for example, see H. Rose, Nucl. Instrum. Meth., A519, 12). Specifically, a spherical aberration corrector using a system having four stages of 6-poles or 4-poles and three stages of 8-poles of magnetic field type arranged alternately or a chromatic/spherical aberration corrector based on an electromagnetic pole of 12 poles has been proposed.
Required for all electromagnetic poles constituting the multiple pole correction system as above are extremely high accuracies in mechanical working and arrangement. Correction of a fine aberration is vulnerable to the influence of noises and power supply variations and there need a variety of highly stable power supplies and adjustment measures. Further, in the magnetic field type using magnetic poles, an issue of magnetization hysteresis arises which is responsible for irregularities in characteristics. Especially, in the case of a projecting magnetic pole as possessed by the aberration corrector, a high permeability material used therefor gives a factor that induces a magnetic noise externally. On the other hand, in the electrostatic system, sophisticated insulating structures oppose a beam in vacuum, giving rise to contamination responsible for a drift the influence of which must be reduced for the purpose of practicing.
Because of the aforementioned problems of difficulties in technique and of high costs, the multiple pole correction system has till now been applied to only some types of electron microscope. In solving the above problems, a laborious technical task of realizing a multiple pole correction system capable of performing highly accurate adjustments inexpensively and easily comes up.